This invention relates to a semiconductor device such as a one-chip microcomputer used in a mobile communication terminal device such as a portable telephone, for example, and more particularly to a semiconductor device having an inherent number storing circuit.
Generally, when inherent identification information such as number data is attached to a semiconductor device, a method for using a semiconductor device with a built-in ROM (Read Only Memory) and storing inherent number data into the ROM cells contained in the semiconductor device at the time of shipment test is used. Therefore, it is considered that the conventional semiconductor device with the built-in ROM has such a construction that the inherent number data can be easily rewritten after shipment of the semiconductor device.
However, for example, in a case where such a semiconductor device with the built-in ROM is used in a portable telephone, the inherent number is used for calculating the telephone rate and the ROM cells are used for storing the inherent number data, then the inherent number data can be freely rewritten after shipment, and as a result, there occurs a possibility that the telephone system may be dishonestly used by intentionally changing the inherent number.
A dishonest-use preventing method using a dual storage technology for providing two ROM devices on the circuit board of the mobile communication terminal and storing data in each ROM device is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-99532.
The above dishonest-use preventing method is intended to prevent dishonest use by utilizing the difficulty of mounting or removing the two ROM devices on or from the circuit board, but the difficulty with the operation is not so serious and the reliability of prevention of dishonest use is relatively low.
As described above, in the semiconductor device having the inherent number storing circuit, the inherent identification data can be relatively easily rewritten after shipment and there occurs a possibility that an applied system device using the semiconductor device may be dishonestly used.